1. Field of the Invention
The present invention relates to an alloy type thermal fuse and a fuse element, and more particularly to those which are useful as a thermoprotector for a battery.
In an alloy type thermal fuse, a low-melting fusible alloy piece to which a flux is applied is used as a fuse element. When such a fuse is used with being mounted on an electric apparatus to be protected and the apparatus abnormally generates heat, a phenomenon occurs in which the low-melting fusible alloy piece is liquefied by the generated heat, the molten metal is spheroidized by the surface tension under the coexistence with the flux that has already melted, and the alloy piece is finally broken as a result of advancement of the spheroidization, whereby the power supply to the apparatus is interrupted.
The first requirement which is imposed on such a low-melting fusible alloy is to have a predetermined melting point which allows the alloy melts at an allowable temperature of the apparatus.
A low-melting fusible alloy is further required to have a narrow solid-liquid coexisting region between the solidus and liquidus lines. In an alloy, usually, a solid-liquid coexisting region exists between the solidus and liquidus lines. In this region, solid-phase particles are dispersed in a liquid phase, so that the region has also the property similar to that of a liquid phase. Consequently, there is the possibility that a low-melting fusible alloy piece is spheroidized and broken in a temperature range (indicated by ΔT) which belongs to the solid-liquid coexisting region. As the solid-liquid coexisting region is wider, the operating temperature of a thermal fuse is more largely dispersed. By contrast, as the solid-liquid coexisting region is narrower, the operating temperature of a thermal fuse is less dispersed, so that a thermal fuse can operate at a predetermined temperature in a correspondingly sure manner. Therefore, an alloy which is to be used as a fuse element of a thermal fuse is requested to have a narrow solid-liquid coexisting region.
Another requirement which is imposed on such a low-melting fusible alloy is that the electrical resistance is low.
When the temperature rise by normal heat generation due to the resistance of the low-melting fusible alloy piece is indicated by ΔT′, the operating temperature is substantially lower by ΔT′ than that in the case where such a temperature rise does not occur. Namely, as ΔT′ is larger, the operation error is substantially larger under the conditions of the same melting point. Therefore, an alloy which is to be used as a fuse element of a thermal fuse is requested to have a low specific resistance. In order to meet the request for reduction of the size of a thermal fuse in accordance with recent tendency of miniaturization of an apparatus, a fuse element of 500 μmφ or less is often used. In such a small fuse element, it is requested to further reduce the specific resistance.
Moreover, a predetermined mechanical strength, particularly a tensile strength is required in order to completely maintain a fuse element against a force such as that (for example, a force acting during a drawing or winding step) which acts on the fuse element during production of the fuse element, that which is applied to the fuse element during a process of producing a thermal fuse, that which is applied to the fuse element during transportation or handling of the thermal fuse, or that which is applied to the fuse element during a heat cycle process).
2. Description of the Prior Art
Conventionally, an alloy containing lead is usually used as a fuse element for an alloy type thermal fuse. However, lead is harmful to the ecological system, and hence not suitable to environment conservation which is a recent global request.
Therefore, it is requested to develop a fuse element which does not contain a metal harmful to the ecological system (Pb, Cd, Tl, or the like). As such a fuse element, a fuse element of a ternary In—Sn—Bi alloy has been proposed.
As a fuse element of a ternary In—Sn—Bi alloy, known are a fuse element which has an alloy composition of 42 to 53% In, 40 to 46% Sn, and 7 to 12% Bi, and in which the operating temperature is 95 to 105° C. (Japanese Patent Application Laying-Open No. 2001-266724), that which has an alloy composition of 55 to 72.5% In., 2.5 to 10% Sn, and 25 to 35% Bi, and in which the operating temperature is 65 to 75° C. (Japanese Patent Application Laying-Open No. 2001-291459), that which has an alloy composition of 0.5 to 10% In, 33 to 43% Sn, and 47 to 66.5% Bi, and in which the operating temperature is 125 to 135° C. (Japanese Patent Application Laying-Open No. 2001-266723), that which has an alloy composition of 51 to 53% In, 42 to 44% Sn, and 4 to 6% Bi, and in which the operating temperature is 107 to 113° C. (Japanese Patent Application Laying-Open No. 59-8229, and that which has an alloy composition of 1 to 15% Sn, 20 to 33% Bi, and the balance In, and in which the operating temperature is 75 to 100° C. (Japanese Patent Application Laying-Open No. 2001-325867).
In a recent portable electronic apparatus such as a portable telephone or a notebook personal computer, a high-energy density secondary battery such as a lithium-ion battery is generally used as a power source, and it is requested to perform thermal protection of the battery by using a thermal fuse. Specifically, because of the high energy density, such a battery generates a large amount of heat in an abnormal state, and hence it is required to interrupt a battery circuit by a thermoprotector before the temperature reaches an abnormal value. As the thermoprotector, a thermal fuse can be preferably used. In such a thermoprotector, a thermal fuse is requested to have an operating temperature of about 100° C. or lower (which is in the vicinity of 100° C. or lower than 100° C.).
When the melting characteristics of a ternary In—Sn—Bi alloy are measured by a DSC (differential scanning calorimeter), a slow transformation c is often observed immediately before a melt end b as shown in FIG. 13 (which shows a DSC curve of 48In-45Sn-7Bi).
In FIG. 13, the amount of the heat energy input to a sample (fuse element) is not changed and the solid phase state is maintained until the temperature reaches a temperature a (solidus temperature); when the temperature exceeds the temperature a, the sample absorbs the heat energy and starts to transform; and, when the temperature exceeds a temperature b (liquidus temperature) and the sample enters the complete liquid phase, the input amount of the heat energy is not changed.
In a usual alloy, such a slow change seldom occurs in the melt end of a DSC curve. A slow change is a special phenomenon in a DSC curve of a ternary In—Sn—Bi alloy.
A slow change in the melt completion of a DSC curve of a fuse element of a ternary In—Sn—Bi alloy causes the width ΔT of the solid-liquid coexisting region to be enlarged. As a result, dispersion of the operating temperature of an alloy type thermal fuse is inevitably increased.